Multi-sensor system for airborne geophysical prospecting and method

ABSTRACT

A multi-sensor electromagnetic (EM) system and method for measuring gradients of EM signals. The multi-sensor EM system includes a frame; a transmitter device attached to the frame and configured to generate a primary EM field; a receiver device attached to the frame and configured to record a secondary EM field generated by the earth after being excited by the primary EM field; and a gradient sensor device attached to the frame and configured to record a gradient of the secondary EM field.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 16/880,360, filed May 21, 2020, which is a divisional of U.S. patent application Ser. No. 15/900,915, filed Feb. 21, 2018, now U.S. Pat. No. 10,768,332, which is a continuation of U.S. patent application Ser. No. 14/966,059, filed Dec. 11, 2015, now U.S. Pat. No. 9,921,331, which is related to, and claims priority from, U.S. Provisional Patent Application Ser. No. 62/092,937 filed Dec. 17, 2014, entitled “APPARATUS FOR AIRBORNE EM SURVEYING USING THREE COMPONENT TRANSMITTERS, RECEIVERS AND GRADIENT SENSORS”, the disclosure of which is incorporated here by reference.

BACKGROUND Technical Field

Embodiments of the subject matter disclosed herein generally relate to methods and systems for generating and/or measuring electromagnetic (EM) fields and, more particularly, to mechanisms and techniques for enhancing detection of near surface, lateral or vertical conductive targets.

Discussion of the Background

EM surveying is a method of geophysical exploration to determine the properties of a portion of the earth's subsurface, information that is especially helpful in the oil and gas industry and the mining industry. EM surveys may be based on a controlled source that generates a primary EM field which carries EM energy into the earth, which induces eddy currents in the earth. The eddy currents generate a secondary EM field or ground response. By measuring the secondary field with an EM receiver, it is possible to estimate the depth and/or composition of the subsurface features. These features may be associated with subterranean hydrocarbon deposits.

A schematic airborne EM survey system 100 generally includes, as illustrated in FIG. 1 , a transmitter 102 for generating the primary EM field 104 that is directed toward the earth. When primary EM field 104 enters the ground 108, it induces eddy currents 106 inside layers 116 and 118 of the earth. These eddy currents 106 generate a secondary electromagnetic field or ground response 110. An EM receiver 112 then measures the response 110 of the ground. Transmitter 102 and receiver 112 may be connected to an aircraft 114 so that a large area of the ground is swept. Receiver 112 may be located concentric and/or coplanar with transmitter 102. A second receiver 113 may also be added.

However, the existing EM surveying systems are not sensitive to near surface formation detection as the signals usually associated with such formation are buried in either ambient or geologic noise. Thus, there is a need to have a new system that is capable of measuring signals associated with near surface formation detection.

SUMMARY

One or more of the embodiments discussed herein illustrate a new multi-sensor EM system that is capable of simultaneously recording not only EM signals associated with the underground formations, but also a gradient of the secondary EM fields.

According to one embodiment, there is a multi-sensor electromagnetic system for measuring EM signals. The multi-sensor EM system includes a frame, a transmitter device attached to the frame and configured to generate a primary EM field, a receiver device attached to the frame and configured to record a secondary EM field generated by the earth after being excited by the primary EM field, and a gradient sensor device attached to the frame and configured to record a gradient of the secondary EM field.

According to another embodiment, there is an electromagnetic surveying system for measuring EM signals. The EM surveying system includes an airborne carrier configured to fly over ground, a multi-sensor EM system configured to generate a primary EM field and to record a secondary EM field generated by the earth as a response to the primary EM field, and a towing system connecting the airborne carrier to the multi-sensor EM system. The multi-sensor EM system is also configured to measure a gradient of the secondary EM field.

According to still another embodiment, there is a method for recording electromagnetic signals during an airborne EM survey. The method includes flying a multi-sensor EM system, generating a primary EM field with a transmitter device attached to a frame of the multi-sensor EM system, recording, with a receiver device attached to the frame, a secondary EM field generated by the earth after being excited by the primary EM field, and recording, with a gradient sensor device attached to the frame, a gradient of the secondary EM field.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. In the drawings:

FIG. 1 is a schematic diagram of an EM acquisition system;

FIG. 2 is a schematic diagram of a multi-sensor EM acquisition system;

FIG. 3 is a general view of the multi-sensor EM system;

FIG. 4 illustrates the X axis coils corresponding to a transmitter, receiver and gradient sensor of the multi-sensor EM system;

FIG. 5 illustrates the transmitter and receiver coil position relative to each other;

FIG. 6 illustrates the arrangement of the transmitter and gradient sensor coils relative to each other;

FIG. 7 illustrates the Y axis coils corresponding to a transmitter, receiver and gradient sensor of the multi-sensor EM system;

FIG. 8 illustrates the Z axis coils corresponding to a transmitter, receiver and gradient sensor of the multi-sensor EM system;

FIG. 9 is a schematic diagram of a multi-sensor EM system towed with an airborne carrier;

FIG. 10 shows the multi-sensor EM system disposed over a hybrid air vehicle;

FIG. 11 is a flowchart of a method for collecting EM data with a multi-sensor EM system; and

FIG. 12 is a schematic diagram of a processing unit that coordinates the transmitter, receiver and gradient sensor devices.

DETAILED DESCRIPTION

The following description of the embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed, for simplicity, with regard to the terminology and structure of a multi-sensor EM system having three-component sensors. However, the embodiments to be discussed next are not limited to such sensors, but they may work with fewer components per sensor.

Reference throughout the specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases “in one embodiment” or “in an embodiment” in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.

According to an embodiment, a multi-sensor EM system includes at least one three-component transmitter and a multi-sensor. The multi-sensor may include a three-component receiver and a three-component gradient sensor. The transmitter and multi-sensors are attached to a platform that is suitable for airborne surveys. In one application, the gradient sensor includes gradient coils placed symmetrically about the transmitter coils to cancel the primary field, because the sum of the two coils will have no primary field. This allows greater sensitivity to the secondary field gradient. Details of the multi-sensor EM system are now discussed with regard to the figures.

FIG. 2 shows one embodiment of a multi-sensor EM system 200 that is designed for airborne geophysical exploration. Multi-sensor EM system 200 includes a rigid frame 202 that hosts a three-component transmitter device 210, a three-component receiver device 220 and a three-component gradient sensor device 230. Frame 202 may be made out of tubes 204 as illustrated in FIG. 3 . Tubes 204 are hollow inside and configured to accommodate one or more wires that form the transmitter, receiver and/or gradient sensor coils. Tubes 204 are connected to each other to form a circle, hexagon or any desired shape. Tubes 204 may be made of plastic or other insulator material. The tubes are light but rigid, so that they do not allow a deformation of the coils located inside the tubes. The tubes are attached to each other as known in the art.

Returning to FIG. 2 , a local control device 240 may be located on the frame 202 and it is electrically connected to the transmitter, receiver and gradient sensor devices. Local control device 240 may communicate with a global control device (not shown, but located, for example, on the airborne carrier) for coordinating the generation of the primary EM fields in the transmitter device and for instructing the receiver and the gradient sensor devices when to record the secondary EM signals. The local control device may be configured to send commands to the transmitter device to sequentially generate magnetic fields in three mutually orthogonal orientations. These magnetic fields may be calculated and generated in such a way that a combination of the fields from multiple transmitters enhances the fields at specific locations and in a specific orientation. This combination has the potential to increase the signal-to-noise thereof, which is desirable. In one embodiment, the local control device is configured to instruct the three-component transmitter device to continuously generate arbitrary waveforms at similar or unique pulse repetition rates. Those skilled in the art would understand that the local and/or global control devices may be configured to instruct the three-component transmitter device to generate any desired shape EM signal.

In addition, the local and/or global control devices may be configured to combine the various EM signals received from the receiver device and the gradient sensor device to enhance the near surface, lateral and/or vertical conductive underground target formations. Such algorithms are known in the art and thus, they are not discussed herein.

Each of the transmitter device 210, receiver device 220 and gradient sensors device 230 is now discussed in detail. FIG. 4 shows the longitudinal components of each of the devices 210, 220 and 230. More specifically, FIG. 4 shows the longitudinal transmitter coil 412X of the transmitter device 210, the longitudinal receiver coil 422X of the receiver device 220, and the longitudinal gradient sensor coil 432X of the gradient sensor device 430. All these components are perpendicular to the X axis, which may be considered to indicate the travel direction of the multi-sensor EM system's carrier. FIG. 4 shows that longitudinal transmitter coil 412X is concentric and between the two longitudinal receiver coils 422X.

The specific arrangement of the receiver coils is called concentric null-coupled receiver and it is illustrated in more detail in FIG. 5 . FIG. 5 shows that the receiver coil 422X has in fact two concentric coils located in a same plane and these two coils are also concentric and located in a same plane with the transmitter coil 412X. Currents flowing through the two concentric receiver coils have opposite directions. This arrangement of the receiver coils contains substantially equal and opposite components of the transmitted field. This provides an overall increase in sensitivity for the secondary fields of interest. The system may also include one or more adjustable components to fine tune the primary field nulling.

FIG. 4 also shows the gradient sensor coil 432X, which includes two parallel elements (coils) 432A and 432B located symmetrically about each transmitter coil 412X. Each coil pair 432A and 432B is wound in series, opposite to each other, as illustrated in FIG. 6 , to provide a signal that is substantially devoid of the primary EM field and ambient noise, but it is sensitive to the spatial gradient of the secondary fields of interest for each axis. Currents flowing through the coil pair 432A and 432B have opposite directions. This allows for detection of near surface structures that might otherwise be buried in either ambient or geologic noise.

FIG. 7 shows the relative positions of the transmitter component 412Y, receiver component 422Y and gradient sensor component 432Y along the Y axis, which is substantially identical to the structure illustrated in FIG. 4 . FIG. 8 shows the Z components 412Z, 422Z and 432Z of the transmitter, receiver and gradient sensor devices, respectively. Note that FIG. 8 also shows some of the tubes 804 forming the frame that supports the coils discussed above.

The structural components (frame 202, tubes 804 and other connecting devices not shown) supporting the three axis transmitters, receivers and gradient coils may be assembled as a stand-alone unit of arbitrary shape to be towed under a carrier. FIG. 9 shows such a system 900 having a carrier 902 that tows with a towing system 904 (e.g., ropes and/or cable) a multi-sensor EM system 906. System 906 includes at least a transmitter device 910, receiver device 920 and gradient sensor device 930 as discussed in the previous embodiments. Carrier 902 may be a light weight helicopter or a lighter than air vehicle or a hybrid air vehicle or any aircraft. A hybrid air vehicle is defined by two components: a propelling mechanism for generating thrust and an enclosure that houses a lighter than the air gas that provides positive buoyancy. In this way, the thrust generated by the propelling mechanism is not used to maintain a given altitude of the hybrid vehicle. Such vehicles can cover a larger physical area resulting in greater sensitivity and low flight speeds, which results in increasing data density and lower overall noise levels (no vibrating engines).

In one embodiment, the multi-sensor EM system is substantially composed of composite structures formed in such a way as to provide fundamentally a symmetric three component transmitter. One transmitter is vertically perpendicular to the flight direction (X), one transmitter is horizontally perpendicular to the flight direction and one transmitter is horizontally parallel to the flight direction. The transmitters may have the same physical area. However, inflight loading related to airflow over the structures limits the practical size of the X and Y transmitters to approximately ⅕ to ½ the area of the Z looking coil. The transmitter device structure may be tubular or a composite 1-beam as disclosed in U.S. Pat. No. 7,646,201, the entire content of which is incorporated herein by reference. The transmitter coils are collocated forming a substantially rigid structure. Typical transmitter pulse repetition frequencies include 30 Hz for each axis, if fired sequentially, or 29.5 Hz, 30.0 Hz and 31.0 Hz in each of the X, Y and Z axis. In some cases, it is possible transmitting similar waveforms on each axis at the same pulse repetition rate. In one embodiment, it may be desirable to independently control the power levels of each axis to direct the primary magnetic field towards a desired target (e.g., along a chosen direction). The power levels may also be controlled in a continuously varying fashion to essentially sweep the resulting magnetic field to ensure maximum coupling of the directed field with a conductive target of arbitrary geometry. The power levels may be controlled by the local and/or global control devices discussed above. This approach may be particularly useful in the field of unexploded ordinance mapping.

FIG. 10 illustrates an embodiment in which carrier 902 includes three transmitting coils 910X, 910Y and 910Z that generate EM signals for penetrating the earth. Carrier 902 is a hybrid air vehicle. Each of the transmitting coils includes a coil of wire extending around a gas envelope 903 of the carrier. Gas envelope 903 is configured to host the lighter than air gas. For example, a first transmitting coil 910X is oriented in a vertical plane extending around the gas envelope of the carrier 902, transversely to the longitudinal axis of the carrier, and thereby, having a dipole moment along the X-axis. Coil 910X may be located generally midway between the nose and the tail of the carrier to maximize its area. However, coil 910X may be located anywhere along the carrier. A second transmitting coil 910Y is oriented in a vertical plane extending around the envelope 903, from the nose to the tail, with a dipole moment on the Y-axis. A third transmitting coil 910Z is oriented in a horizontal plane extending around the gas envelope of the carrier, from the nose to the tail, with a dipole moment on the Z-axis.

According to another embodiment, the transmitting coils 910X-Z are generally elliptical in shape. Alternatively, the transmitting coils can be of other shapes that can be accommodated by the carrier. In one embodiment, the coils follow the shape of the envelope 903. Receiver coils 920X-Z, which measure the ground response due to the excitation from the transmitting coils, may be located next to the transmitting coils, as illustrated in FIG. 10 . The use of one or more three-component receiver(s) 920, along with the orientations of the transmitting coils 910X-Z thereby allow collection of data in three different orthogonal directions, being nominally vertically perpendicular to the direction of flight, horizontally parallel to the direction of flight and horizontally perpendicular to the direction of flight. The gradient sensor device 930 may also be located on the carrier. For simplicity, FIG. 10 shows only the X components 930A and 930B of the gradient sensor device 930.

The multi-sensor EM system discussed herein takes 12 measurements of the EM field for each transmitter firing sequence. These measurements advantageously can be combined to enhance near surface, lateral or vertical conductive targets. However, in one embodiment, the multi-sensor EM system may have less than 12 measurements, i.e., either one of the transmitter, receiver and gradient sensor devices may have less than three components. For example, it is possible to build a single axis system that includes just gradient coils. It is also possible to build a system that includes a transmitter and a concentric null coupled receiver, which is consistent in function with all other TDEM systems assuming a Z orientation.

According to still another embodiment, an additional payload that can be carried by hybrid air vehicle 902, as compared to the payload of a traditional aircraft or helicopter, may also be used to carry a large auxiliary power unit (not shown) with sufficient electrical capacity to increase the output power of one or more of the transmitter coils, e.g., by a factor of twenty or more. For example, the transmitter power of the transmitter device 910 may be as much as 40,000,000 amp-metres square (Am²) whereas the most powerful system currently mounted on helicopter or aircraft is in the order of 2,000,000 Am². The null-coupled receiver device allows for increases in transmitter moment without sacrificing receiver sensitivity.

A method for recording secondary magnetic fields with an airborne multi-sensor EM system is now discussed with regard to FIG. 11 . The method includes a step 1100 of flying a multi-sensor EM system, a step 1102 of generating a primary EM field with a transmitter device attached to a frame of the multi-sensor EM system, a step 1104 of recording, with a receiver device attached to the frame, a secondary EM field generated by the earth after being excited by the primary EM field, and a step 1106 of recording, with a gradient sensor device attached to the frame, a gradient of the secondary EM field. The multi-sensor EM system may be any of the systems discussed above.

There are many possible implementations of the multi-sensor EM system discussed above. The multi-sensor EM system may include, in addition to the components discussed above, many peripheral sensors to determine the position or orientation or state of the electromagnetic measurement, such as a Global Positioning System (GPS), radar or laser altimeter, gyroscopes or inclinometers measuring transmitter or sensor positions, thermometers, or other sensors measuring other geophysical data (such as radar or laser for topography, gravity or gradiometers sensors, spectrometer sensors, magnetometers to measure the ambient earth magnetic field, etc.). Consequently, there are also many different methods to record, process, combine and control all of these signals and sensors.

As also will be appreciated by one skilled in the art, the exemplary embodiments may be embodied in a processing unit 1200, as illustrated in FIG. 12 . Processing unit 1200 includes a processor 1202 that is connected through a bus 1204 to a storage device 1206. Processing unit 1200 may also include an input/output interface 1208 through which data can be exchanged with the processor and/or storage device. For example, a keyboard, mouse or other device may be connected to the input/output interface 1208 to send commands to the processor and/or to collect data stored in storage device or to provide data necessary to the processor. In one application, the processor combines the 12 measurements for generating, for example, a raw image of the underground formations. Also, the processor may be used to calculate, for example, the positions of the coils. Results of this or another algorithm may be visualized on a screen 1210. The method discussed above may be implemented in a wireless communication device or in a computer program product. Accordingly, the exemplary embodiments may take the form of an entirely hardware embodiment or an embodiment combining hardware and software aspects. Further, the exemplary embodiments may take the form of a computer program product stored on a computer-readable storage medium having computer-readable instructions embodied in the medium. Any suitable computer-readable medium may be utilized, including hard disks, CD-ROMs, digital versatile discs (DVD), optical storage devices or magnetic storage devices such as a floppy disk or magnetic tape. Other non-limiting examples of computer-readable media include flash-type memories or other known types of memories.

For greater clarity, the figures used to help describe the invention are simplified to illustrate key features. For example, figures are not to scale and certain elements may be disproportionate in size and/or location. Furthermore, it is anticipated that the shape of various components may be different when reduced to practice, for example. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims. Those skilled in the art would appreciate that features from any embodiments may be combined to generate a new embodiment.

The disclosed embodiments provide a method and multi-sensor EM system capable of recording gradient EM signals indicative of a near surface formation, in addition to the traditional EM signals. It should be understood that this description is not intended to limit the invention. On the contrary, the exemplary embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the exemplary embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details.

Although the features and elements of the present exemplary embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein.

This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims. 

What is claimed is:
 1. A multi-sensor electromagnetic (EM) system for measuring earth EM response to an EM excitation while the multi-sensor EM system is flown over a surveyed area, the multi-sensor EM system comprising: a transmitter device configured to generate the EM excitation; a receiver device configured to detect and record an EM field generated by the earth in response to the EM excitation; a gradient sensor device configured to record a gradient of the EM field, wherein a transmitter coil of the transmitter device and at least one receiver coil of the receiver device are arranged substantially in a plane, and the gradient sensor device includes two coils placed out of the plane.
 2. The system of claim 1, wherein the two coils of the gradient sensor device are placed substantially parallel with and on either side of the plane.
 3. The system of claim 2, wherein the two coils of the gradient sensor device are placed substantially at same distance from the plane.
 4. The system of claim 1, wherein the two coils of the gradient sensor are electrically connected and configured to have current flowing in opposite directions.
 5. The system of claim 1, wherein the receiver device has two receiver coils coplanar with the transmitter coil, one of the two receiver coils surrounding the transmitter coil and another of the two receiver coils being surrounded by the transmitter coil.
 6. The system of claim 5, wherein the two receiver coils are electrically connected and configured to have current flowing in opposite directions.
 7. The system of claim 1, further comprising a peripheral sensor configured to determine a location and/or an orientation of at least one of the transmitter device, the receiver device and the gradient sensor.
 8. The system of claim 7, wherein the peripheral sensor includes a global positioning system.
 9. The system of claim 7, wherein the peripheral sensor includes at least one of a radar, laser altimeter, gyroscope, or inclinometer.
 10. The system of claim 1, further comprising: a controller electrically connected to the transmitter device, the controller controlling each of three transmitter coils of the transmitter device to generate a magnetic field with a given pulse repetition frequency.
 11. The system of claim 1, wherein the transmitter device includes three transmitter coils perpendicular to each other, the receiver device includes three receiver coils perpendicular to each other, and the gradient sensor device includes three pairs of gradient coils perpendicular to each other, the transmitter coil and the at least one receiver coil of the receiver device arranged substantially in the plane, being one of the three transmitter coils and of the three receiver coils, respectively, and having one of the three pairs of gradient coils being the two coils placed out of the plane.
 12. The system of claim 11, wherein each of the three pairs of gradient coils have coils arranged parallel to one another, on either side of a respective one of the three receiver coils.
 13. The system of claim 12, wherein coils of any of the three pairs of gradient coils are electrically connected so that a current flowing in one of the coils has opposite direction to a current flowing in another one of the coils.
 14. The system of claim 1, further comprising a support structure configured to maintain relative positions of coils of the transmitter device, of the receiver device and of the gradient sensor while the multi-sensor EM system is flown over a surveyed area.
 15. A multi-sensor electromagnetic (EM) system for measuring earth EM response to an EM excitation while the multi-sensor EM system is flown over a surveyed area, the multi-sensor EM system comprising: a transmitter device configured to generate the EM excitation; a receiver device configured to detect and record an EM field generated by the earth in response to the EM excitation; a gradient sensor device configured to record a gradient of the EM field, wherein the transmitter device, the receiver device and the gradient sensor device are configured to generate the EM excitation, detect the EM field and the gradient of the EM field on three non-coplanar directions, wherein, for at least one of the three directions, coils of the transmitter device and of the receiver device are coplanar and two coils of the gradient sensor device are placed symmetrically on either side and parallel with a plane of the coils of the transmitter device and of the receiver device.
 16. The multi-sensor electromagnetic (EM) system of claim 15, wherein the three non-coplanar directions are perpendicular to one another.
 17. The multi-sensor electromagnetic (EM) system of claim 15, wherein the two coils of the gradient sensor device are electrically connected and configured to have current flowing in opposite directions.
 18. The multi-sensor electromagnetic (EM) system of claim 15, further comprising: a support structure configured to maintain an arrangement of the transmitter device, the receiver device and the gradient sensor while the multi-sensor EM system is flown over the surveyed area; and a peripheral sensor configured to determine location and/or orientation of at least one of the transmitter device, the receiver device and the gradient sensor while the multi-sensor EM system is flown over the surveyed area.
 19. A method of geophysical exploration using a multi-sensor electromagnetic (EM) system for measuring earth EM response to an EM excitation, the method comprising: flying a multi-sensor EM system over a surveyed area; controlling a transmitter device of the multi-sensor EM system to generate an EM excitation; recording an EM field generated by the earth in response to the EM excitation, using with a receiver device of the multi-sensor EM system; and recording a gradient of the EM field with a gradient sensor device of the multi-sensor EM system, wherein the transmitter device, the receiver device and the gradient sensor device are configured to generate the EM excitation, to detect the EM field and the gradient of the EM field, respectively, on three directions perpendicular to one another, wherein, for at least one of the three directions, coils of the transmitter device and of the receiver device are coplanar and two coils of the gradient sensor device are placed symmetrically on either side and parallel with a plane of the coils of the transmitter device and of the receiver device. 